Semiconductor manufacturing, such as the fabrication of integrated circuits, typically entails the use of photolithography. A semiconductor substrate on which circuits are being formed, usually a silicon wafer, is coated with a material, such as a photoresist, that changes solubility when exposed to radiation. A lithography tool, such as a mask or reticle, positioned between the radiation source and the semiconductor substrate casts a shadow to control which areas of the substrate are exposed to the radiation. After the exposure, the photoresist is removed from either the exposed or the unexposed areas, leaving a patterned layer of photoresist on the wafer that protects parts of the wafer during a subsequent etching or diffusion process.
The photolithography process allows multiple integrated circuit devices or electromechanical devices, often referred to as “chips,” to be formed on each wafer. The wafer is then cut up into individual dies, each including a single integrated circuit device or electromechanical device. Ultimately, these dies are subjected to additional operations and packaged into individual integrated circuit chips or electromechanical devices.
During the manufacturing process, variations in exposure and focus require that the patterns developed by lithographic processes be continually monitored or measured to determine if the dimensions of the patterns are within acceptable ranges. The importance of such monitoring, often referred to as process control, increases considerably as pattern sizes become smaller, especially as minimum feature sizes approach the limits of resolution available by the lithographic process. In order to achieve ever-higher device density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting metallization lines, spacing and diameter of contact holes and vias, and the surface geometry such as corners and edges of various features. Features on the wafer are three-dimensional structures and a complete characterization must describe not just a surface dimension, such as the top width of a line or trench, but a complete three-dimensional profile of the feature. Process engineers must be able to accurately measure the critical dimensions (CD) of such surface features to fine tune the fabrication process and assure a desired device geometry is obtained.
Typically, CD measurements are made using instruments such as a scanning electron microscope (SEM). In a scanning electron microscope (SEM), a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary beam. The secondary electrons are detected, and an image is formed, with the brightness at each point of the image being determined by the number of secondary electrons detected when the beam impacts a corresponding spot on the surface. As features continue to get smaller and smaller, however, there comes a point where the features to be measured are too small for the resolution provided by an ordinary SEM.
Transmission electron microscopes (TEMs) allow observers to see extremely small features, on the order of nanometers. In contrast to SEMs, which only image the surface of a material, TEM also allows analysis of the internal structure of a sample. In a TEM, a broad beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. Samples are typically less than 100 nm thick.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the substrate are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface.
As semiconductor geometries continue to shrink, manufacturers increasingly rely on transmission electron microscopes (TEMs) for monitoring the process, analyzing defects, and investigating interface layer morphology. The term “TEM” as used herein refers to a TEM or a STEM, and references to preparing a sample for a TEM are to be understood to also include preparing a sample for viewing on an STEM. Because a sample must be very thin for viewing with transmission electron microscopy (whether TEM or STEM), preparation of the sample can be delicate, time-consuming work.
Thin TEM samples cut from a bulk sample material are known as “lamellae”. Lamellae are typically less than 100 nm thick, but for some applications a lamella must be considerably thinner. With advanced semiconductor fabrication processes at 30 nm and below, a lamella needs to be less than 20 nm in thickness in order to avoid overlap among small scale structures. Currently, thinning below 30 nm is difficult and not robust. Thickness variations in the sample result in lamella bending, overmilling, or other catastrophic defects. For such thin samples, lamella preparation is a critical step in TEM analysis that significantly determines the quality of structural characterization and analysis of the smallest and most critical structures.
Even though the information that can be discovered by TEM analysis can be very valuable, the entire process of creating and measuring TEM samples has historically been so labor intensive and time consuming that it has not been practical to use this type of analysis for manufacturing process control. The use of focused ion beam (FIB) systems to create lamellae for TEM microscopy is known in the art. FIB systems are capable of milling lamella sufficiently thin to be used in a TEM system. The use of dual-beam systems for TEM sample preparation is known in the art. A dual-beam system has a FIB column for milling a lamella from a bulk sample and a SEM column for imaging the lamella, typically as the lamella is being milled. Dual-beam systems improve the time required to prepare samples for TEM analysis. While the use of FIB methods in sample preparation has reduced the time required to prepare samples for TEM analysis down to only a few hours, it is not unusual to analyze 15 to 50 TEM samples from a given wafer. As a result, speed of sample preparation is a very important factor in the use of TEM analysis, especially for semiconductor process control.
FIG. 1A shows a prior art FIB system in an orientation for performing initial milling on a bulk sample material to create a sample lamella for TEM analysis. The bulk sample material, substrate 108, is loaded into sample stage 106 of the tool. Substrate 108 is oriented so that its top surface is perpendicular to focused ion beam 104 emitted from FIB column 102. Most of the ion beam machining done to create lamella 110 is performed with substrate 108 and FIB column 102 in this orientation. Due to the focusing (i.e., a convergent conical shape) and the path of ion beam 104, this perpendicular milling causes lamella 110 to be tapered from top to bottom. That is, lamella 110 is thinner at the top than it is at the bottom. Furthermore, lamella 110 remains securely attached to substrate 108 at boundary 114. Lamella 110 must be removed from substrate 108 before it can be used in the TEM. In addition, material removed from substrate 108 while milling with ion beam 104 in the vertical orientation may be redeposited on or flow onto the face of lamella 110, forming amorphous layer 112. Amorphous layer 112 reduces the quality of the TEM analysis and must be removed or polished away before lamella 110 can be used with the TEM.
FIG. 1B shows a prior art FIB system in a tilted orientation for post-processing a sample lamella using overtilting, polishing, and/or undercutting. Overtilting is the process of removing the taper from the sides of lamella 110 to make the faces of lamella 110 substantially parallel. Polishing is the process of removing amorphous layer(s) 112 from lamella 110 that collected on lamella 110 from the previous initial milling. Undercutting is the process of partially or fully detaching lamella 110 from substrate 108 at or near boundary 114. Prior art lamella-creation tools orient FIB column 102 so that ion beam 104 is in the vertical orientation during the initial machining of substrate 108 (i.e., normal to the top surface of substrate 108). After the initial machining of substrate 108, in order to perform the processes of overtilting, polishing, and undercutting, the sample must be tilted away from a position that is perpendicular to ion beam 104 in both directions so that the additional ion milling can be performed. Either sample stage 106 or FIB column 102 is rotated an angle 116 about the long axis of lamella 110. That is, either sample stage 106 or FIB column 102 is rotated an angle 116 relative to a plane defined by the long axis of lamella 110 and the normal to the top surface of substrate 108. Put another way, sample stage 106 or FIB column 102 is rotated about an axis that is perpendicular to the sheet of FIG. 1A and located within the cross-section of lamella 110 shown in FIG. 1A, preferably near the center of the cross-section of lamella 110.
In the architectures known in the prior art, either sample stage 106 or FIB column 102 must be tilted about an axis that is perpendicular to the plane defined by FIB column 102 and a normal to the top surface of substrate 108 after the initial milling to perform any required post-processing on lamella 110. The provision of either of these tilts (i.e., stage or column tilt) to the tool is complex and adds to the expense, maintenance, and fragility of the tool. In prior art systems, if FIB column 102 is held in a fixed position throughout the entire lamella preparation processes, then sample stage 106 must have five degrees of freedom: translation in the X, Y, and Z directions, rotation about the axis perpendicular to the top surface of the substrate, and rotation about the axis perpendicular to FIB column 102. Alternatively, if sample stage 106 is made to have only four degrees of freedom (X, Y, Z, and rotation about the axis perpendicular to the sample's top surface), then FIB column 102 must rotate with respect to the rest of the tool during milling to perform overfilling, polishing, and undercutting.
A TEM sample preparation system having a sample stage 106 and/or FIB column 102 that can be tilted accurately within an acceptable limits of drift (on the order of nanometers) is complex, expensive, and requires additional maintenance. A TEM sample preparation system having a sample stage 106 with only four degrees of freedom and a FIB column 102 that remains in a fixed position would, all other things being equal, cost less than the tools described above, be easier to assemble and maintain, and be less likely to break down. It is thus desirable to be able to perform angled milling with FIB column 102 without having to tilt the sample with respect to ion beam 104 during processing.
Additionally, lamellae formed using the prior art methods described above are subject to an undesirable side effect known as “curtaining.” FIG. 2 shows a sample 200 exhibiting the curtaining effect. When substrate 108 is formed from a heterogeneous structure (e.g, metal gates and shields along with silicon and silicon dioxide), ion beam 104 differentially mills different elements at different mill rates. Some metal elements tend to shadow the lighter material underneath them. For example, sample 200 comprises silicon portion 202 and tungsten portion 204. Silicon portion 202 is milled at a higher reate than tungsten portion 204. The resulting effect is a rippled lamella face, or curtain 206, which is not milled back as far in the areas of metal as it is milled in the areas without metal. This effect is called “curtaining” because the rippled features on the lamella face resemble a hanging curtain. When the ion beam is directed vertically (i.e., perpendicular to the top surface of the substrate), the curtaining effect is most pronounced. Curtaining artifacts reduce the quality of the TEM imaging and limit the minimal useful specimen thickness. For ultra-thin TEM samples, the two cross-section faces are in very close proximity so thickness variations from curtaining effects can cause a sample lamella to be unusable. Thus, it is desirable to reduce curtaining artifacts during the preparation of TEM sample lamellae.
Although the foregoing description of the process for lamella preparation has been presented in the context of semiconductor fabrication, lamella preparation for other applications is also now a common practice. For example, in biological imaging, it is often advantageous to create lamellae from resin-embedded or cryogenically-frozen samples of cells or tissues. A TEM or STEM is then used to image these lamella, thereby gaining information about various cellular ultrastructures.
In addition, the micro- and nanomachining procedures described above in the context of lamella preparation may also be employed in other nanofabrication procedures such as MEMs fabrication and other processes for the creation of mechanical, electrical, and electromechanical devices, especially in cases where these structures span size ranges from tens of micrometers to nanometer sizes.
The beam positioning and tilting procedures described above in the context of focused ion beams may also have application for other types of microfabrication processes, for example, the use of waterjet cutters and laser beams.